1. Field of the Invention
The present invention relates to isolators and communication apparatuses. Particularly, the present invention relates to an isolator that is smaller than known isolators, and a communication apparatus including such an isolator.
2. Description of the Related Art
A lumped-constant isolator is a high-frequency component that transmits signals in direction of transmission while blocking signals in the opposite direction. A lumped-constant isolator is used, for example, in a transmission circuit of a mobile communication apparatus such as a cellular phone. Generally, an isolator includes a magnetic plate composed of ferrite or the like, a common electrode disposed on a first surface of the magnetic plate, a plurality of center conductors crossing each other on a second surface of the magnetic plate, matching capacitors respectively connected to the center conductors, and a terminating resistor connected to one of the center conductors. Since the matching capacitors require high Q factors in order to reduce insertion loss, single-plate capacitors have been used, as disclosed in U.S. Pat. No. 6,420,941.
Recently, as the functions of cellular phones are enhanced, a demand has been raised for miniaturization of isolators.
In order to achieve miniaturization of isolators while maintaining operating frequencies, the balance between the inductances of center conductors (hereinafter denoted as L) and the capacitances of matching capacitors (hereinafter referred to as C) must be considered. More specifically, miniaturization of magnetic plates is necessary for miniaturization of isolators. Thus, the lengths of center conductors become shorter, and the inductance L decreases accordingly. Particularly, when the inductance L of center conductors connected to input/output terminals becomes lower, the capacitance C of the capacitors must be increased. This, however, increases insertion loss of the isolator.
Furthermore, in order to increase the capacitance C of a single-plate capacitor, the size of the capacitor must be increased or the thickness of the capacitor must be reduced. However, the increase in the size of the capacitor is against the demand for miniaturization of the isolator, and the reduction in the thickness of the capacitor makes the capacitor more susceptible to damage. As an alternative, a multilayer capacitor that is smaller than a single-plate capacitor can be used, as disclosed in British Patent No. 2,350,238. However, generally, a multilayer capacitor has a low Q factor, and insertion loss of the isolator considerably increases.
Thus, in a proposed arrangement, a magnetic plate has a substantially rectangular shape as viewed in plan, and center conductors connected to input/output terminals are disposed along diagonal directions of the magnetic plate to maximize the lengths of the center conductors, maintaining the inductance L of the center conductors L to be high and reducing the capacitance C of the capacitors.
However, since a center conductor connected to the terminating resistor is disposed along a width direction of the magnetic plate, the inductance L of the center conductor is small. Thus, the capacitance C of a capacitor connected to the center conductor must be high. In a conventional isolator, a single-plate capacitor is used as a capacitor for a terminating side. Thus, a large capacitor must be used in order to increase the capacitance C. This has been a main factor that inhibits miniaturization of an isolator.